<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>

</head>


<button id="btn">change</button>
<div id="demo" ></div>




<body>
<style>
    #demo{width: 100px;
        height: 100px;
        border: 1px solid;}


</style>
<script>
    function $(id){
        return document.getElementById(id);
    }
    function show(){
        $('demo').style.display='block';

    }
    $('btn').onclick=show;

    btn.onclick = function () {
        demo.style.border = 'double red';
    }
</script>



</body>
</html>